Introduction– An intense light interaction with a quantum system
can excite the system's electrons towards extreme nonequilibrium states
during a fraction of its cycle [1]}, [2]}. Excited by the
wavefield and subjected to the internal forces inside the system, the
electrons emit coherent electromagnetic (EM) radiation that can contain from
tens to many hundreds of harmonics of an incident light [3]}, [4]}. This is one of the fundamental processes in the
intense laser-matter interaction called high harmonic generation (HHG) [5]}. The HHG process in atoms or molecules with the three-step model
[6]} explanation is a well-demonstrated method for producing
coherent extreme ultraviolet radiation. The coherent spectrum of HHG implies
access to the extreme time resolution of the underlying quantum dynamics
that opens the way for attosecond physics [7]}, [8]} and ultrafast
imaging methods for emitters themselves. In particular, using HHG
spectroscopy one can reconstruct the crystal potential [9]},
observe Mott [10]} and Peierls [11]} transitions, retrieve
the band structure [12]}, [13]}.
